The invention relates to a tunable semiconductor diode laser with distributed reflection comprising a semiconductor body in which a first radiation-conductive layer is present on a first passive layer, in which radiation-conductive layer a strip-shaped resonator cavity is formed between two surfaces extending substantially perpendicularly to the said layers and within which are juxtaposed a first section having a first current supply and an associated active region with a p-n junction which at a sufficiently high current strength in the forward direction generates coherent electromagnetic radiation which active region is present within the amplification profile of the radiation-conductive layer, a second section having a second current supply by which the refractive index of a part of the radiation-conductive layer present within the said section can be varied, and a third section having a third current supply, in which the part of the resonant cavity present within the said section comprises a periodic variation of the refractive index in the longitudinal direction.
Such a semiconductor diode layer is described in the article "1.55.sub./ .mu.m wavelength tunable FBH-DBR laser"by Y. Kotaki et al in Electron. Lett. 23 (1987) 327.
Semiconductor diode lasers of a variety of constructions are used in many fields. The resonant cavity can be realized in various manners. In many cases it is formed by two mirror surfaces extending parallel to each other, for which customarily cleavage faces of the semiconductor crystal are used. By repeated reflection against the mirror surfaces radiation modes known as Fabry-Perot (FP) modes are generated.
According to another embodiment the resonant cavity is obtained by a periodic variation of the effective refractive index for the generated radiation along at least a part of the length of the resonant cavity. Instead of reflection against mirror surfaces, reflection at a grating (formed by the said periodic refractive index variation) is used. Lasers in which this is the case are termed lasers with distributed feedback (DFB(=Distributed Feed Back)lasers). They exist in various constructions and are known as "distributed feedback" (DFB) and "Distributed Bragg Reflection" (DBR) lasers, of which the semiconductor diode laser of the first-mentioned article is an example. In the former the section in which the periodic refractive index variation is present coincides substantially with the section in which the active region is present, while in the latter these sections are substantially entirely separated. As compared with the first mentioned Fabry-Perot lasers. Both types of lasers have inter alia the advantage that they can more easily oscillate in one single stable longitudinal mode of oscillation ("single longitudinal mode" or SLM mode) and that within a wide temperature range and at a high output power. This is of importance in particular when used in optical telecommunication since in SLM mode the chromatic dispersion is minimum so that the signal can be transported in a disturbance-free manner over a larger distance through the optical glass fiber. For heterodyne and coherent optical glass fiber communication the tunability of the wavelength of a semiconductor diode laser to be used as a transmitter or as a local oscillator in a receiver is a necessary condition. For such an application the semiconductor diode lasers of the DBR type mentioned in the opening paragraph are excellently suitable. The section with the periodic refractive index variation-- hereinafter to be termed Bragg section-- in this laser comprises a separate current supply with which the refractive index in said section can be varied so that the Bragg condition and hence the wavelength of the semiconductor diode laser varies. This occurs independently of for example, the output power of the semiconductor diode laser which is determined by the current flowing through the section in which the active region with the p-n junction is situated-- hereinafter termed the active section-- . The Bragg condition is determined by the following equation EQU .lambda.=2*n.sub.R *.DELTA.tm (1) wherein .lambda. is the wavelength of the radiation generated by the semiconductor laser, n.sub.R is the effective refractive index of the section with the periodic refractive index variation (which depends on the current strength in the Bragg section) and .DELTA. is the period of the periodic refractive index variation. The phase condition for oscillation of such a semiconductor diode laser is: EQU .theta..sub.R +.theta..sub.L =2*N*.pi. (2)
wherein .theta..sub.R is the phase of the radiation which returns from the Bragg section viewed from the active section, .theta..sub.L is the phase of the radiation which returns from the direction of the active section to the Bragg section and N is an integer with which the mode in which the semiconductor diode laser oscillates is characterized. The occurring vibration mode is that mode in which the reflectivity in the Bragg section is maximum. In other words that mode the phase .theta..sub.R of which is as close as possible to .pi./2 will be the mode in which the laser oscillates. Associated therewith is a wavelength determined by the Bragg condition (compare 1). When the Bragg wavelength is varied by current supply in the Bragg section and hence the refractive index in the Bragg section varies, a situation arises in which the phase of another mode becomes situated as closely as or even closer to .pi./2 than the phase of the original mode. As a result of this the so-called mode oscillation or mode jumping is formed as a result of which the wavelength also starts oscillating, which is undesired. In order to counteract this a further section-- hereinafter to be termed phase section-- is present in the known semiconductor diode laser and has a separate current supply with which the refractive index in said section and hence the phase .theta..sub.L of the radiation returning from the direction of the active region can be varied in such a manner that the mode in which .theta..sub.R is as close as possible to .pi./2, remains equal to the original mode. In this connection it does not matter whether the phase section is present between the Bragg section and the active section or that the phase section and the Bragg section are present on each side of the active section. In both cases tunability over a large wavelength range and within a certain mode of operation is obtained.
Experiments have demonstrated that a disadvantage of the known semiconductor diode laser is that it is not continuously tunable throughout its wavelength range within a mode of oscillation. As a matter of fact it has been found that so-called forbidden zones may occur in the wavelength range, i.e. zones in which oscillation within the given mode occurs in no single pair of currents by the Bragg and phase sections.